a lot=very much 是副词短语，修饰动词
吃的多 睡大觉 多谢
eat a lot
sleep a lot
thanks a lot
a lot of=lots of是形容词短语，修饰名词
许多学生 许多朋友 许多钱 许多人
a lot of students(可数名词)
a lot of money (不可数名词)
a lot of friends (可数名词)
a lot of people (不可数名词)
